This invention relates to a guide for a file, and more particularly to a guide for use in sharpening the teeth on a chain saw.
In sharpening the teeth of a chain saw, it is desirable that the teeth be sharpened at a particular predetermined angle from the direction of movement of the teeth and, whatever angle is selected, especially important that all of the teeth be sharpened at the same angle. For these purposes, various file guides have been devised for guiding a file for manual sharpening of the teeth of a chain saw. Such file guides are typically mounted on the bar of the chain saw and guide the file at a predetermined angle relative to the cutting edge of each tooth on the chain. Certain of these file guides, such as those shown in U.S. Pat Nos. 3,744,349 and 3,322,000, also have a tooth stop for holding each tooth, in the desired position relative to the file, so that each tooth can be sharpened to a predetermined depth. A problem with the prior art guides has been their failure to guide the file and to position the tooth to be sharpened relative to the file with sufficient accuracy and consistency to ensure that all the teeth on the chain are sharpened at the same angle and to the same depth. A chain saw having teeth with cutting edges sharpened at differing angles will not cut through a piece of wood in a straight line but, rather, will cut in a curved line, because chain saws turn while cutting toward the side of the bar having the greater number of sharply angled teeth. In an extreme case, the chain saw will bind against the sides of the cut in the wood. If a chain saw has teeth that are not sharpened to a uniform depth, the saw will not run smoothly, in that some teeth will cut relatively deeply into the wood, while other teeth cut lightly, if at all, into the wood. A problem with the prior art file guides using reciprocal files, such as the file guide in U.S. Pat. No. 3,322,000, has been abrasion arising from movement of the file past the guide member. Thus, either the file abrades away the guide, thereby changing the sharpening angle and/or depth of sharpening, or the file is dulled by the guide, thereby reducing the useful life of the file. A problem with the prior art file guides using a rotary file, such as the file guide in U.S. Pat. No. 3,744,349, is that the replacement rotary files for the guides are relatively expensive specialty items.